1. Field of the Invention
A method and apparatus for analyzing magnets and/or actuator coils in magnetic head assemblies for magnetic disk drives, and specifically a method and apparatus for measuring the torque per unit current generated by magnets as a function of position or angle in actuator coils in hard disk drives.
2. Discussion of the Background
Hard disk drives contain disk media (i.e., platters on which data is written and from which the data is read), actuators which move the head over the spinning disk media, and drive electronics which control the positioning of the actuator over the disk media. Radial actuators use a pivot to swing the head in an arcing path, and linear actuators move the head in a linear path. UGIMAG, Inc, the assignee of the present application, manufactures permanent magnets and magnet assemblies for use in actuators. As shown partially in FIGS. 1A and 1B, in a voice coil motor (VCM) type electromagnetic rotary actuator, the actuator includes at least one permanent magnet 84, having north and south polarizations, and an actuator coil 50 with a number, N, of windings, which generates a magnetic field when current is applied to the windings. Often the permanent magnet 84 is mounted on a metal mounting plate 82 to form a plate assembly 80B. The plate 82 is typically made of steel or another highly magnetizable material. The actuator may also contain a second plate assembly 80A on the opposite side of the actuator coil 50 as compared to the plate assembly 80B. In FIG. 1B, the plate assembly 80A is not illustrated to provide a clearer view of the actuator coil 50 with reference to the polarity of the magnet 84. Based on the magnitude and direction of the current in the actuator coil 50, the actuator coil 50 is biased toward one side or the other of the permanent magnet, producing a torque on the actuator arm. The torque or force is produced by the interaction of the current in the coil with the magnet 84 of the plate assemblies 80A and 80B. This is known as the Lorentz force.
In rotary actuators, since the actuator coil 50 is mounted on an actuator arm near the arm's pivot point, the opposite end of the actuator arm that supports the disk drive's read-write head is moved in the opposite direction over the disk media. The current is supplied by a servo-control system in response to commands from a computer control system. The torque is applied to the assembly to move the heads from track to track over the recording surface. Currents are supplied that accelerate and decelerate the angular motion in order to achieve the desired motion.
One of the design parameters of an actuator system is the torque produced on the actuator arm per unit current. For proper control of the position, this torque per unit current, often called the torque constant K.sub.t, should be nearly constant over the usable angular range of the actuator. For rapid repositioning of the read-write heads, the torque constant should be high enough so that the necessary torque can be applied with currents available from the servo-controller. In a practical actuator, the torque constant K.sub.t is not constant. Often K.sub.t is maximum near the center of the angular range and decreases as the arm approaches either limit. Designers specify the properties of the function K.sub.t (.theta.) where .theta. is the angular position of the actuator arm referenced to a suitable origin.
A variety of methods have been used to test manufactured magnet assemblies for quality control purposes. Most of the methods use very simple Hall probe measurements of the induction in the gap of the magnet assembly at a few selected locations. Sometimes, additional locations are tested to indicate possible errors in the position of the magnets or in the location of the transition zone of bi-polarized magnets. Static flux measurements have also been used to measure the induction in the gap of the magnet assembly.
These known techniques compare measurements to the theoretically determined induction at selected points or areas when using flux measurements, but the techniques do not correlate to the properties of the function K.sub.t (.theta.). There have been several attempts to measure torque directly with a torque transducer or indirectly by measuring the force on a transducer at a certain position on the actuator arm when a known current is applied to the coil. While these systems work, they require complex alignment, almost continuous calibration, and do not isolate the magnetic contribution to the torque from mechanical effects such as friction or torque from the current carrying leads. The most complex systems use two force transducers and can measure the torque at only three angles in about one minute.
One known system measures K.sub.t (.theta.) indirectly by analyzing the signals in the disk drive servo controller. The actuator assembly is mounted in the disk drive housing which is mounted on a rotary stage. This allows the measurements to be carried out as a function of angle. Calibration is done by comparing the measurement at one angle to the torque produced by a standard mass. The test takes about forty-five minutes. The initial alignment of the disk drive housing and actuator requires about fifteen minutes. The system further requires an expensive signal analyzer. This system also requires a complex interface to the disk drive servo electronics and is very slow.
A second torque measurement system is available from Vibrac Corporation of Amherst, N.H. The system measures torque from an applied current as a function of angle by using a torque transducer. A measurement over an actuator's range of motion takes up to thirty minutes. A third torque measurement system, the M-15 Universal Torque Tester, is available from Measurement Research, Inc. (MRI) of San Fernando, Calif. This system automatically measures torque in the clockwise and counterclockwise directions due to an applied current derived from force measurements. To measure over the entire range takes about two minutes.
In the second and third systems, contributions from friction and forces on current terminals are indistinguishable from the magnetic component in the torque data, and these systems are difficult to set up and operate. Calibration is difficult, with the results being operator-dependent and incompatible with standards from the National Institute for Standards and Technology (NIST). Further, because of the testing time required for these systems, these systems are inappropriate for even medium to large scale production volumes.